scsi: allow arbitrary LUNs

scsi: allow arbitrary LUNs

This only requires changes in two places: in SCSIBus, we need to look
for a free LUN if somebody creates a device with a pre-existing scsi-id
but the default LUN (-1, meaning "search for a free spot"); in vSCSI,
we need to actually parse the LUN according to the SCSI spec.

For vSCSI, max_target/max_lun are set according to the logical unit
addressing format in SAM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>